Many coatings, adhesives and sealants are formed by effecting the polymerization of a suitable monomer. The physical properties of the final product, the stability of the reagents in storage and the rate of curing under a given set of conditions can be greatly effected by the choice of curing agent that is utilized for crosslinking. Many monomers/resins have epoxide functional groups and/or unsaturated bonds that can participate in polymerization and crosslinking. Resins with epoxide functional groups are very useful due to their ability to bond together metals and many other substances extremely well. They form strongly adherent coatings and are widely used in applications as diverse as gluing together aircraft parts to coating electronic circuit boards.
Curing agents that themselves become a part of the polymerized product, usually by forming crosslinkages, are also known as hardeners. The extent of such crosslinkages and their nature often control the bulk properties, such as resistance to tensile or shear stress, water resistance and the like, of the polymerized product.
An important class of curing compounds have amine functional groups. Epoxy groups react at ambient temperatures with primary aliphatic amines to form secondary amines and with secondary amines to form tertiary amines. Reaction rates depend on epoxy and amine structure and concentration. Terminal epoxy groups, such as glycidyl ether and esters, are generally more reactive than internal epoxy groups, which are more sterically hindered. Reactivity of amines tends to increase with base strength and decrease with steric crowding. Generally, aliphatic amines are more reactive than aromatic amines, which are much less basic.
Often times, functional groups such as amines, in particular primary aliphatic amines, are too reactive and difficult to control in reactions with epoxide monomers at ambient temperatures. Thus, two package systems are commonly employed so that the epoxide resin is stored separately from the amine-curing agent. This results in the need for dual container packaging and limited pot life.
The structure of the amine-curing agent, e.g., the length of the molecule and the number of amine functional groups available on each molecule, affect the final properties of the product. In addition, the ratio of the curing agent to the monomers and the presence of fillers, if any, also affects the final product. For instance, curing agents with aromatic amines may produce a product with better thermal properties while curing agents comprising aliphatic poly-amines may provide products with superior flexibility. Other properties of interest are adhesive strength, especially in the use of epoxy resins, peel strength, tensile and shear strength, hardness and scratch resistance, nature of materials bonded, thermal expansion, creep, heat resistance, water resistance, dielectric strength, strains introduced due to the curing process, and, of course, cost, color and compatibility.
Amine-curing agents are available in a variety of forms for different applications. Therefore, there is a need for curing agents that retain the advantages of the availability of, and the experience gained with, amine-curing agents while permitting greater control over storage and the rate of curing. Preferably, such improved curing agents should be available at a low cost and in a variety of structural forms.
One of the desirable alternative functional groups for the purpose of affording control over the curing rate and storage properties of curing agents is the thiol group because its reaction rates can be readily controlled. The use of curing agents with thiol functional groups (thiol-curing agents) is known. However, unlike amine-curing agents, synthesis limitations make it difficult to make a wide variety of curing agents with thiol functional groups. Furthermore, preparation of curing agents with thiols also results in the release of offensive odors.
U.S. Pat. No. 3,318,974 discloses a method for curing unsaturated polyesters with peroxides in the presence of thioglycolic acid. Another U.S. Pat. No. 3,291,716 describes the use of thiol containing compounds in the curing of epoxy resins. Thus, while curing unsaturated resins and epoxide resins with thiol containing compounds is described, the choice of such compounds is limited due to difficulties in efficient synthesis. Although mixtures of thiol containing compounds (e.g. trimethylol propane trimercaptopropionate) with amines are known curing agents, there is no substantial reaction between these compounds to produce thioglycolamides. That is, they co-cure as separate entities with the epoxide.
It is an object of the present invention to overcome the drawbacks of amine-curing agents while retaining the advantages associated with amine-curing agents by making possible suitable thiol-curing agents.
It is an objective of the present invention to provide a method for preparing curing agents that cure through thiol groups.
It is another objective of the invention to provide a method for preparing a curing agent with thiol groups from compounds having at least one amine group.
It is a further object of the invention to provide curing agents whose rate of curing can be controlled to provide desired properties such as viscosity, storage and extent of curing.